Method for producing seamless metallic tube by cold rolling

ABSTRACT

In cold rolling of a seamless metallic tube with a cold pilger mill, when a reduction ratio of an outside diameter becomes excessive in comparison to a reduction ratio of a wall thickness, strain in a circumferential direction on flange regions becomes excessive. As a result, compressive stress in a circumferential direction becomes excessive, so that wrinkle imperfections are generated inside a tube and folded on groove bottom regions to become folded defects. This process is repeated to be developed into inside defects. When a hollow shell is produced by the Mannesmann-mandrel process instead of the extrusion process, inside wrinkle imperfections are generated with a sizing mill (a stretch reducer or a sizer) at a stage of hollow shell. Since the inside wrinkle imperfections further grow in cold rolling, it is necessary to pay attention thereto. In consideration of these, (a reduction ratio of an outside diameter/a reduction ratio of a wall thickness both in a cold pilger mill) is set to not more than 1/2. It is, thereby, possible to obtain a high-quality seamless metallic tube, inside defects thereof being suppressed, produced by cold rolling.

TECHNICAL FIELD

The present invention relates to a cold rolling method for a seamlessmetallic tube, particularly to a method for producing a high-qualityseamless metallic tube by cold rolling for the purpose of ensuringinside-surface quality of high-grade specialty tubes from a viewpoint ofsuppressing wrinkle imperfections on the inside surface.

BACKGROUND ART

When a seamless metallic tube does not satisfy specific requirements inquality, strength, or dimensional accuracy in an as-hot-finishedcondition, it is subjected to a cold working process. Commonly knowncold working processes are a cold drawing method with a die and a plugor a mandrel bar, and a cold rolling method with a cold pilger mill.

Since available reduction rate for tube material is extremely high incold rolling with a cold pilger mill, the cold rolling has advantages asfollows: about ten-times elongation is possible by rolling; an excellenteffect on correcting eccentric wall thickness of tube can be exhibited;a diameter-reducing process is not required; and no yield loss isgenerated.

Meanwhile, the cold rolling with a cold pilger mill has a drawback ofextremely low productivity in comparison to the cold drawing method. Thecold rolling with a cold pilger mill is, therefore, mainly suitable forcold working of high-grade specialty tube such as stainless steel tubeand high-alloy steel tube that requires expensive raw materials andcostly intermediate treatments.

FIG. 1 is a view for illustrating a mechanism of the cold rolling with acold pilger mill. In a cold rolling method with a cold pilger mill, ahollow shell 1 is processed between a pair of rolls 2 and a taperedmandrel bar 4 to perform a diameter-reducing rolling for the hollowshell 1, so as to obtain a rolled tube 5. Each roll 2 has acircumferential length-wise tapered groove caliber 3 decreasinggradually in diameter along the circumferential length. The taperedmandrel bar 4 decreases gradually in diameter along a longitudinaldirection.

That is, the groove caliber 3 is formed along a circumference of each ofpaired rolls 2 of the cold pilger mill, and the groove caliber becomesnarrower/smaller with the progress of rotation of rolls 2. The rolls 2repeat forward and backward strokes along the tapered mandrel bar whilebeing rotated by driven roll shafts 2 s so that the hollow shell 1 isrolled between the rolls 2 and the mandrel bar 4 to perform adiameter-reducing rolling of the hollow shell 1 (see Non PatentLiterature 1, for example).

FIG. 2 is an explanatory view showing a working principle of coldrolling with a cold pilger mill FIG. 2A shows a working state at a startpoint of a forward stroke, and FIG. 2B shows a working state at a startpoint of a backward stroke. As shown in FIG. 2, in the cold pilger mill,according to an outside diameter and a wall thickness (do and to in thefigure) of a hollow shell 1 and an outside diameter and a wall thickness(t and d in the figure) of a product, selectively adopted are a pair ofrolls 2 each having a tapered groove caliber 3 which decreases graduallyin diameter from an engaging entry side of the rolls toward a finishingexit side thereof, and a tapered mandrel bar 4 which decreases alsogradually in diameter from an engaging entry side toward a finishingexit side, and forward and backward strokes are repeated to reduce awall thickness of the hollow shell 1 while reducing a diameter thereof

The hollow shell 1 is turned by about 60° and is given a feed of about 5to 15 mm at a start point of the forward stroke in reciprocation motionof the cold pilger mill, so that a new portion of the hollow shell isrolled, which is repeated.

There are two types of cold pilger mills: a rolling mill developed by“MANNESMANN-DEMAG the rolling mill for reducing wall thickness in bothforward and backward strokes; and a rolling mill developed by BLAWKNOX,the rolling mill for reducing wall thickness only in a forward stroke.The former is commonly used for rolling stainless steel tube, high-alloymetallic tube, or zirconium tube, while the latter is used for rollingan aluminum tube, aluminum-alloy tube, copper tube, and copper-alloytube.

CITATION LIST

NON PATENT LITERATURE

[NON PATENT LITERATURE 1]

The Iron and Steel Institute of Japan, “3rd Edition Iron and SteelHandbook, Vol. III (2), Steel Bars/Steel Pipe/Facilities Commonly Usedfor Rolling”, Nov. 20, 1980, Pages 1183-1189

SUMMARY OF INVENTION

Technical Problem

Since characteristic of high-quality is strongly demanded for high-gradespecialty tube subjected to cold rolling with a cold pilger mill, it isnecessary to suppress generation of inside-surface defects resultingfrom inside-surface wrinkle imperfections on a tube as a product afterthe cold rolling. There, however, has heretofore been no proposal on amethod for producing a high-quality seamless steel tube, whereininside-surface defects are inhibited from occurring in the cold rollingwith a cold pilger mill.

The present invention is achieved in view of the above problem, and anobject of the present invention is to propose a method for producing ahigh-quality seamless steel tube by the cold rolling with a cold pilgermill.

Although a cold pilger mill performing rolling in both forward andbackward strokes (MANNESMANN-DEMAG) will be described for theexplanation of the present invention, objects of the present inventionare not limited to this type but can be applied to a cold pilger millreducing wall thickness only in a forward stroke (BLAWKNOX).

SOLUTION TO PROBLEM

In order to solve the above problem, the present inventor found fromvarious examinations the following. That is, in cold rolling of aseamless metallic tube with a cold pilger mill, when a reduction rate ofan outside diameter becomes excessive in comparison to a reduction rateof a wall thickness, circumferential compressive stress imposed on ahollow shell becomes excessive, and wrinkle imperfections are, hence,easily generated on the tube inside surface.

Furthermore, when a hollow shell is produced by the Mannesmann-mandrelmill process instead of the Ugine-Sejournet extrusion process, insidewrinkle imperfections may be generated with a sizing mill (a stretchreducer or a sizer) at a stage of hollow shell. The inside wrinkleimperfections generated at the stage of hollow shell significantlyinfluence quality of a high-grade specialty tube subjected to the coldrolling with a cold pilger mill

The present invention is completed based on the above knowledge, and agist thereof is methods of the following (1) and (2) each for producinga seamless metallic tube by cold rolling.

(1) A method for producing a seamless metallic tube by cold rolling witha cold pilger mill, comprising the steps of: when elongating a hollowshell in such a manner that an outside diameter thereof is reduced whilereducing a wall thickness thereof, according to outside diameters andwall thicknesses of the hollow shell and a rolled tube as a product,selectively using a pair of rolls and a tapered mandrel, the rolls eachhaving a tapered groove caliber which decreases gradually in diameterfrom an engaging entry side of roll toward a finishing exit sidethereof, the tapered mandrel bar decreasing also gradually in diameterfrom an engaging entry side toward a finishing exit side; and setting areduction rate Rd of the outside diameter to not more than one-half of areduction rate Rt of the wall thickness.

The reduction rate Rd of outside diameter and the reduction rate Rt ofwall thickness are calculated by following expressions (a) and (b):

Rd={1-(d/do)}×100 (%)  (a)

Rt={1(t/to)}×100 (%)  (b)

wherein

do: outside diameter of hollow shell, d: finishing outside diameter, to:wall thickness of hollow shell, and t: finishing wall thickness.

(2) In the method of (1) for producing a seamless metallic tube by coldrolling, it is preferable to use a hollow shell subjected to a hotreducing mill process with a stretch reducer under the condition that areduction rate of outside diameter is not more than 77%. Or it ispreferable to use a hollow shell subjected to a hot reducing millprocess with a sizer under the condition that a reduction rate ofoutside diameter is not more than 33%.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the method of the present invention for producing aseamless metallic tube by cold rolling, it is possible to suppressgeneration of inside-surface defects resulting from inside-surfacewrinkle imperfections by improving a working balance between a reductionrate Rd of outside diameter and a reduction rate Rt of wall thickness atthe time of the elongation-rolling which accompanies the reduction ofthe diameter while reducing the wall thickness. It is, therefore,possible to ensure high-quality for the product after cold rolling.

Furthermore, when a hollow shell is produced by the Mannesmann-mandrelmill process, it is possible to further improve product quality aftercold rolling by limiting a reduction rate of outside diameter in asizing mill (a stretch reducer or a sizer).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A view for illustrating a mechanism of the cold rolling with acold pilger mill.

FIG. 2 An explanatory view showing a working principle of the coldrolling with a cold pilger mill, FIG. 2A shows a working state at astart point of a forward stroke, and

FIG. 2B shows a working state at a start point of a backward stroke.

FIG. 3 A view showing a divided model of a cross-section of a tuberolled with a cold pilger mill

FIG. 4 A view showing deformation behaviors of a cross-section of a tuberolled with a cold pilger mill

FIG. 5 A view for illustrating one example of production steps in theMannesmann-mandrel mill process for hot-producing seamless steel tube.

DESCRIPTION OF EMBODIMENTS

FIG. 3 is a view showing a segmentation model of a cross-section of anin-process tube rolled during rolling with a cold pilger mill. Thecross-section of tube can be segmented into groove bottom regions 11, 14and flange regions 12, 13 based on whether or not the inner surface oftube 1 is in contact with a mandrel bar 4. The groove bottom regions 11,14 are elongated by being subjected to a wall thickness reduction workby means of the rolls and the mandrel bar 4, and the flange regions 12,13 are deformed by being pulled by the elongation of the groove bottomregions. That is, metals of the groove bottom regions 11, 14 aredeformed under external pressure, internal pressure, and axialcompression force, and metals of the flange regions 12, 13 are deformedunder external force and axial force. of tension

FIG. 4 is a view showing deformation behaviors with respect to thecross-section of tube during rolling with a cold pilger mill. FIG. 4Ashows a deformation behavior during rolling in a forward stroke (forwardrolling), and FIG. 4B shows a deformation behavior during rolling in abackward stroke (backward rolling). The deformation behaviors shown inFIG. 4 are based on a working pattern in which the tube 1 is turned onlyin the forward stroke rolling and not turned in the backward strokerolling. That is, when seen from tube side, the rolls are turnedrelative to the in-process tube to be repositioned only during theforward stroke but not relatively turned during the backward stroke.

As shown in FIGS. 4A and 4B, in a type of cold pilger mill performingrolling in both forward and backward strokes (MANNESMANN-DEMAG), A turnof 60° is basically adopted. Deformation of the cross-section of tubeis, thus, not symmetrical but asymmetrical to a horizontal axis and avertical axis of a groove caliber. In deformation behaviors shown inFIG. 4, segments 11 and 14 indicate the groove bottom regions, andsegments 12 and 13 indicate the flange regions in the i-th forwardstroke rolling.

In the deformation behaviors shown in FIG. 4, when a reduction rate Rdof outside diameter is excessively large relative to a reduction rate Rtof wall thickness, compressive strain φθ in a circumferential directionon the flange regions is increased. As a result, compressive stress σθin a circumferential direction (not shown) becomes excessive, so thatinside-surface wrinkle imperfections are generated and folded on thegroove bottom regions. This process is repeated to be developed intoinside-surface defects, resulting in deterioration of inside-surfacequality.

In the production of a specialty tube which requires a high level ofquality characteristic, a ratio of the reduction rate Rd of outsidediameter to the reduction rate Rt of wall thickness determines qualityof product. Furthermore, when a hollow shell to be processed with a coldpilger mill is produced by the Mannesmann-mandrel mill process insteadof the hot extrusion (Ugine-Sejournet extrusion) process, the hollowshell includes inside-surface wrinkle imperfections generated in a hotreducing mill process. The inside-surface wrinkle imperfections furtherencourages the development thereof and affects the cold rolling process.

FIG. 5 is a view for illustrating one example of production steps in theMannesmann-mandrel mill process for hot-producing seamless steel tube.In this process, a solid round billet 21 heated to a predeterminedtemperature serves as a starting material to be rolled. This roundbillet 21 is fed to a piercing mill 23, and a central portion thereof ispierced so as to produce a hollow piece (hollow shell) 22. Next, theproduced hollow piece 22 is directly fed to a successive elongatingdevice, which is a mandrel mill 24, to be elongated, so as to obtain ahollow shell 22.

At the time of the elongation rolling with the mandrel mill 24, amaterial temperature of the hollow shell 22 is lowered with a mandrelbar 24 b inserted into the inside of the hollow shell and rolling rolls24 r for constraining outer surface of the hollow shell. Therefore, thehollow shell 22 rolled in the mandrel mill 24 is then placed into are-heating furnace 25 to be re-heated. After that, the hollow shell goesthrough a sizing mill such as a stretch reducer 26 or a sizer (notshown) and becomes a hot-rolled seamless steel tube. When a temperaturedrop in the mandrel mill is small, the re-heating furnace is notrequired.

However, in the stretch reducer or the sizer for performing the sizingmill process in the above Mannesmann-mandrel mill process, the hollowshell 22 goes through rolling rolls 26 r to be finished by a reducingmill process for an outside diameter without using the inside surfaceconstraining tool such as a mandrel bar. Wrinkle imperfections are,thus, easily generated on the inner surface of the hot-finished steeltube.

The present inventor, therefore, performed rolling tests in which, astest specimens, not only hollow shells hot-extruded but also hollowshells subjected to a reducing mill process with a stretch reducer and asizer are used. The rolling tests varying in reduction rate of outsidediameter in a reducing mill process and varying in reduction rates ofoutside diameter along with wall thickness in cold rolling areperformed. Macroscopic structure observations for the specimens areconducted to investigate conditions for suppressing wrinkleimperfections.

As described above, in the cold rolling of a seamless metallic tube witha cold pilger mill, when a reduction rate of outside diameter becomesexcessive in comparison to a reduction rate of wall thickness, strain ina circumferential direction on the flange regions becomes excessive. Asa result, compressive stress in a circumferential direction becomesexcessive, so that wrinkle imperfections are generated on the insidesurface of tube and folded on the groove bottom regions to become foldedimperfections. This process is repeated to be developed into detrimentalinside surface defects.

As a result of the above investigation, when a hollow shell is producedby the Mannesmann-mandrel mill process instead of the hot extrusionprocess, inside-surface wrinkle imperfections may be generated with asizing mill (a stretch reducer or a sizer) at a stage of hollow shell.And when these inside-surface wrinkle imperfections are present, theinside wrinkle imperfections further encourage the development thereofin cold rolling, to which attention shall be paid.

With respect to the method of the present invention for producing aseamless metallic tube by cold rolling, taking into consideration thatnot only hot-extruded hollow shell but also hollow shell made by a hotsizing mill process are to be used, it is necessary to set a reductionrate of outside diameter to not more than one-half of a reduction rateof wall thickness in a cold pilger mill.

In the method of the present invention for producing a seamless metallictube by cold rolling, when a sizing mill process is performed with astretch reducer, it is preferable to use a hollow shell made by a hotreducing mill process under the condition that a reduction rate ofoutside diameter is not more than 77%. Or when a sizing mill process isperformed with a sizer, it is preferable to use a hollow shell made by ahot reducing mill process under the condition that a reduction rate ofoutside diameter is not more than 33%.

EXAMPLES

As test specimens, hollow shells produced by the hot extrusion(Ugine-Sejournet extrusion) process and hollow shells produced by theMannesmann-mandrel mill process (finished with a stretch reducer and asizer) are used. Inside-surface quality of product was evaluated forsamples that underwent cold working with a cold pilger mill fordiameter-reducing rolling

Example 1

A 25Cr-30Ni-3Mo high-alloy steel tube having an outside diameter of 50.8mm and a wall thickness of 5.5 mm produced by the hot extrusion processwas used as a hollow shell for a test specimen. The hollow shell wasrolled with a cold pilger mill to reduce to 38.1 mm in outside diameterand to 2.4 mm in wall thickness. The hollow shell was fed and turned atthe start point of each forward stroke. Test conditions are summarizedbelow.

Diameter of tapered mandrel bar: dm varying from 39.6 to 33.1 mm(tapered)

Feed in forward stroke: f=8.0 mm

Turn angle in forward stroke: θ=60°

Hollow shell dimension: do×to=50.8 mm×5.5 mm

Finishing dimension: d×t=38.1 mm×2.4 mm

Ratio between diameters before and after reduction: d/do=0.75

Elongation ratio: to (do-to)/t (d-t)=2.91

Wall thickness/Outside diameter: t/d=0.063

Reduction rate of outside diameter/Reduction rate of wall thickness:Rd/Rt=0.46<1/2

wherein

Reduction rate of outside diameter: Rd={1-(d/do)}×100 (%)

Reduction rate of wall thickness: Rt={1-(t/to)}×100 (%)

Since no wrinkle imperfections were generated on the hollow shellproduced by the extrusion, generation of inside-surface defectsresulting from the wrinkle imperfections was extremely few on a productafter cold rolling, and satisfactory inside-surface quality wasobtained.

Example 2

A 25Cr-30Ni-3Mo high-alloy steel tube having an outside diameter of 48.6mm and a wall thickness of 6.0 mm produced by the Mannesmann-mandrelmill process with an inclined roll type piercing mill, a mandrel mill,and a stretch reducer was used as a hollow shell for a test specimen.The hollow shell was rolled with a cold pilger mill to reduce to 41.0 mmin outside diameter and to 2.2 mm. in wall thickness The reduction rateof outside diameter in the stretch reducer was not more than 77%. Testconditions are summarized below.

Diameter of mandrel bar: dm=36.4 mm (without taper)

Feed in forward stroke: f=8.0 mm

Turn angle in forward stroke: θ=60°

Hollow shell dimension: do×to=48.6 mm×6.0 mm

Finishing dimension: d×t=41.0 mm×2.2 mm

Ratio between diameters before and after reduction: d/do=0.84

Elongation ratio: to (do-to)/t (d-t)=3.0

Wall thickness/Outside diameter: t/d=0.054

Reduction rate of outside diameter/Reduction rate of wall thickness:Rd/Rt=0.25<1/2

wherein

Reduction rate of outside diameter: Rd={1-(d/do)}×100 (%)

Reduction rate of wall thickness: Rt={1-(t/to)}×100 (%)

While the reduction rate of outside diameter in the stretch reducer wasnot more than 77%, generation of inside wrinkle imperfections wasextremely suppressed since a reducing mill process was performed whileimparting maximum inter-stand tensional force by full-stretch setup.Also generation of inside-surface defects resulting from the wrinkleimperfections was, thus, mild on a product after cold rolling, andsatisfactory inside-surface quality was obtained.

Example 3

A 25Cr-30Ni-3Mo high-alloy steel tube having an outside diameter of101.6 mm and a wall thickness of 7.0 mm produced by theMannesmann-mandrel mill process with an inclined roll type piercingmill, a mandrel mill, and a sizer was used as a hollow shell for a testspecimen. The hollow shell was rolled with a cold pilger mill to reduceto 88.9 mm in outside diameter and to 2.8 mm in wall thickness. Areduction rate of outside diameter in the sizer was not more than 33%.Test conditions are summarized below.

Diameter of mandrel bar: dm=83.8 mm (without taper)

Feed in forward stroke: f=10.0 mm

Turn angle in forward stroke: θ=60°

Hollow shell dimension: do×to=101.6 mm×7.0 mm

Finishing dimension: d×t=88.9 mm×2.8 mm

Ratio between diameters before and after reduction: d/do=0.88

Elongation ratio: to (do-to)/t (d-t)=2.8

Wall thickness/Outside diameter: t/d=0.032

Reduction rate of outside diameter/Reduction rate of wall thickness:Rd/Rt=0.21<1/2

wherein

Reduction rate of outside diameter: Rd={1-(d/do)}×100 (%)

Reduction rate of wall thickness: Rt={1-(t/to)}×100 (%)

Since the reduction rate of outside diameter in the sizer was not morethan 33%, which was considerably small in comparison to the reductionrate of outside diameter in case of the stretch reducer, generation ofinside-surface wrinkle imperfections was extremely suppressed.Generation of inside-surface defects resulting from the wrinkleimperfections was, thus, mile on a product after cold rolling, andsatisfactory inside-surface quality was obtained.

INDUSTRIAL APPLICABILITY

According to the method of the present invention for producing aseamless metallic tube by cold rolling, it is possible to suppressgeneration of inside-surface defects resulting from inside wrinkleimperfections by improving a working balance between a reduction rate Rdof outside diameter and a reduction rate Rt of wall thickness at thetime of elongation rolling accompanying diameter reduction working whilereducing wall thickness. It is, therefore, possible to obtain ahigh-quality tube as a product after cold rolling.

Furthermore, when a hollow shell is produced by the Mannesmann-mandrelmill process, it is possible to further improve the product qualityafter cold rolling by limiting a reduction rate of outside diameter in asizing mill (a stretch reducer or a sizer). The present invention, thus,can be widely applied as a method for producing a high-quality seamlessmetallic tube by cold rolling.

REFERENCE SIGNS LIST

1: Hollow shell

2: Groove caliber roll

3: Tapered groove caliber

4: Tapered mandrel bar

5: Rolled tube

11, 14: Segment on groove bottom side

12, 13: Segment on flange side

21: Round billet

22: Hollow piece, hollow shell

24: Mandrel mill

25: Re-heating furnace

26: Sizing mill, stretch reducer

1. A method for producing a seamless metallic tube by cold rolling witha cold pilger mill, comprising the steps of: when elongating a hollowshell in such a manner that an outside diameter thereof is reduced whilereducing a wall thickness thereof, selectively using a pair of rolls anda tapered mandrel bar according to outside diameters and wallthicknesses of the hollow shell and a rolled tube as a product, therolls each having a tapered groove caliber which decreases gradually indiameter from an engaging entry side of roll toward a finishing exitside thereof, the tapered mandrel bar decreasing also gradually indiameter from an engaging entry side toward an finishing exit side; and,setting a reduction rate Rd of outside diameter to not more thanone-half of a reduction rate Rt of wall thickness, whereinRd={1-(d/do)}×100 (%) Rt={1-(t/to)}×100 (%) do: outside diameter ofhollow shell d: finishing outside diameter to: wall thickness of hollowshell t: finishing wall thickness.
 2. The method for producing aseamless metallic tube by cold rolling according to claim 1, whereinused is a hollow shell made by a hot reducing mill process with astretch reducer under the condition that a reduction rate of outsidediameter is not more than 77%.
 3. The method for producing a seamlessmetallic tube by cold rolling according to claim 1, wherein used is ahollow shell made by a hot reducing mill process with a sizer under thecondition that a reduction rate of outside diameter is not more than33%.